With the recent technological advances in compression coding, cryptography, communication, and so forth, it has become common for digitized content data of movies and music pieces to be transmitted and received via networks. Since such content data are often protected by copyright, a variety of techniques have been developed for combating unauthorized devices which are likely to perform operations leading to copyright infringement such as illegal copying.
For example, in High-bandwidth Digital Content Protection (HDCP), prior to transmission or reception of content data, the validity of devices connected to an HDCP system is examined using identification information for identifying HDCP compliant devices. Once the validity is confirmed, encrypted content data is transmitted and received.
HDCP is a sort of Digital Rights Management (DRM) technique and is intended to combat unauthorized copying by encrypting output of image content and video content including high-definition television (Hi-Vision) content. More particularly, HDCP protects the last stage in the distribution process, encrypting content transmitted over digital interfaces from sources (e.g., set-top boxes, DVD players, personal computers, game consoles, etc.) to sinks (e.g., display devices such as high definition TVs, etc.).
For example, referring to FIG. 7, HDCP authentication is performed when a source (e.g., video player, digital tuner) and a sink (e.g., monitor) are connected using an High-Density Multimedia Interface (HDMI) cable or a Digital Visual Interface (DVI) cable, which transmits Hi-Vision content such as digital terrestrial broadcast content and Blue-ray disk content.
More specifically, as illustrated in FIG. 8, after receiving key selection vector (KSV) of the source (AKSV), the sink has to calculate an R0′ value within a desired time, e.g., 100 ms, from the time that the source finishes writing (sending) its AKSV to the sink. For the source, after receiving BSKV, it is necessary to calculate an R0 value within 100 ms from the time that the sink writing (sending) its key selection vector (BKSV) to the source.
Thus, the R0 value and the R0′ value are compared 100 ms+αlater than the time when the source wrote (sent) its AKSV (a is an estimated time period, such as a few milliseconds, from AKSV writing to BKSV reception). In this way, authentication is completed.
Meanwhile, in the above case, the physical distance between the source (e.g., video player, digital tuner) and the sink (e.g., monitor) connected by an HDMI cable or a DVI cable is very short. However, along with the widespread use of home LANs (local area networks) and IPTV (Internet Protocol television) systems, there are more occasions where the distances between sources and sinks are extended by transmission media such as an IP (Internet Protocol) network.
For example, in a case where a Digital Living Network Alliance (DLNA) network is constructed using a home LAN or where an in-house video delivery system is designed, an arrangement illustrated in FIG. 9 may be employed. In this arrangement, video data is converted into IP packets using a video relayer device to make it possible to transmit the video data using an IP network regardless of the distance between a source and a sink. There are techniques developed for transmitting video data using networks.